1. Field of Invention
This invention relates to a method for increasing the selectivity of a liquid phase oxidation of isobutane with molecular oxygen for the production of tertiary butyl hydroperoxide.
2. Prior Art
Winkler et al U.S. Pat. No. 2,845,461 describes the preparation of tertiary butyl hydroperoxide by the oxidation of liquid isobutane with molecular oxygen at 200.degree.-300.degree. F. at a pressure exceeding 400 psig.
Grane U.S. Pat. No. 3,907,202 describes the liquid phase oxidation of isobutane at 200.degree.-300.degree. F. at 300-700 psig using a feed containing 0.2 to 3% of isopropyl alcohol, isobutyl alcohol or secondary butyl alcohol. Said Grane patent explains that small amounts of tertiary butyl alcohol, such as up to 3% of tertiary butyl alcohol "gives substantially no improvement in the selectivity of reaction toward the production of tertiary butyl hydroperoxide."
Said Grane patent explains that the liquid oxidate (after removal of unreacted isobutane or IB) contains Tertiary Butyl Hydroperoxide (or TBHP) and Tertiary Butyl Alcohol (or TBA) as the principal components. As the selectivity for TBHP decreases, larger amounts of TBA are found in the oxidate. The selectivity for TBHP is decreased by increasing residence time and/or temperature. Increasing the residence time in the reactor increases the extent of conversion of the IB. Hence maintaining uniform residence time in a series of tests eliminates the effect of this highly important variable. A relatively small change in temperature, while retaining a uniform residence time in a series of tests, provides a measurable change in selectivity as well as a measurable change in conversion.
It is convenient to employ the same pressure throughout a series of comparative runs, but the effect, if any, of pressure on selectivity and/or conversion is relatively small compared to the significant effects of temperature. The pressure must be sufficient to maintain liquid phase conditions, and desirably is as high (while still being under a safety limit of about 1,000 psig) as economically tolerable. Thus laboratory tests can conveniently be conducted at higher pressures (e.g. 800 psig) than would be used industrially.
Heretofore there have been a variety of studies of the chronology of the liquid phase oxidation of isobutane so that there has long been a recognition of the propensity of tertiary butyl hydroperoxide to be somewhat autocatalytic. That is, as more tertiary butyl hydroperoxide is formed during the oxidation of isobutane, the presence of such tertiary butyl hydroperoxide tends to accelerate the reaction rate, serving like a catalyst for more rapid conversion of additional isobutane, but with the formation of larger amounts of byproduct tertiary butyl alcohol. Relatively large proportions of TBA are formed when most of the isobutane is converted at an elevated temperature. The relative proportion of TBA formation increases when there are incremental increases in the TBHP product concentration at such elevated temperature high conversion conditions. Hence, it is sometimes appropriate to control both the residence time and the temperature of the isobutane oxidation so that about 20-40% of the isobutane is converted prior to the separation of the reaction product for the recycling of the isobutane and for the separation of the liquid oxidate. Said Winkler et al patent describes and claims selectivities of about 50 mol percent. As the selectivity for TBHP is increased, the concentration of TBA in a Winkler type oxidate is decreased.
The market for tertiary butyl alcohol has long been complicated by the availability of a few outlets at prices which were attractively high, but which were also relatively small volume markets and not readily susceptible of expansion. A large market for isobutyl alcohol in fuel oil or in other liquid fuels has long existed, but the price at which tertiary butyl alcohol can be sold as fuel has ordinarily been relatively unattractive. In recent years, a considerable portion of the tertiary butyl alcohol byproduct from processes involving TBHP has been disposed of in gasoline at a price significantly less than would be expected for a chemical having the unique advantages of tertiary butyl alcohol.
A high price and a relative scarcity of isobutane has resulted from the complicated changes in the petrochemical marketing of recent years. The supply and demand factors have been such that there have been periods and localities in which there appeared to be incentives for the conversion of tertiary butyl alcohol to isobutylene and the hydrogenation of isobutylene to isobutane for recycling through the reactor for the formation of tertiary butyl hydroperoxide. Such conversion of TBA to IB permits a plant to market propylene oxide as its sole product and to generally ignore some of the fluctuations in the demand or price for TBA. The selectivity of the oxidation step for preparing larger amounts of tertiary butyl hydroperoxide and relatively smaller amounts of tertiary butyl alcohol is always important, but can be emphasized when costs are considered for a process in which the tertiary butyl alcohol is recycled by the steps of dehydration followed by hydrogenation. The fluctuating market price of tertiary butyl alcohol reflects both its utility as a special chemical and its utility as a component of gasoline.
There are important advantages to preparing propylene oxide by the use of tertiary butyl hydroperoxide as an intermediate. Such propylene oxide production must compete, however, with methods for preparing propylene oxide by the chlorohydrin method or by procedures using ethyl benzene hydroperoxide and/or other materials for which the market for the coproduct may be more attractive than is the established market for tertiary butyl alcohol.
During the more than a decade since the filing of the application maturing as said Winkler patent, there has been a continuing demand for improvements in the selectivity for the reaction of IB for the formation of TBHP, but the technologists and engineers have been unable to achieve as high selectivity for TBHP as for some other tonnage chemicals.